I haven't used sitebuilder in awhile (switched most to the icloud, but if you are in edit mode, and you click on the menu there should be some options that pop up. Let me go in and see if I have an old site where I can find it and I will get back to you.

OK, Click on the menu -- a side window pops up and the controls for how many buttons you have is under "effects." You can also control the width of the whole menu under BASICS. You may have to make some manual adjustments to tjhe length of your menu and maybe make the text on the buttons a tiny bit smaller. Hope that helps.

You should be able to drag the menu to the left a little to line it up better, but you do need to uncheck the lock position and size box on the right. Click the menu and you should see it near the bottom of the properties editor.